The present invention relates to a receiving system for free-space optical communications.
Receiving systems for free-space optical communications are arranged to receive an electromagnetic signal propagated through the atmosphere, rather than through a fiber-optic communications cable. Consequently, the electromagnetic signal at optical frequencies (e.g., infrared frequency range) is 10 subject to attenuation from atmospheric conditions, such as rain, snow, temperature inversion, or precipitation. The attenuation limits the maximum path length between an optical transmitter and an optical receiving system. In some cases, limitations in the maximum path length for optical communications systems may lead to the use of competitive technologies, such as short-haul microwave radios, instead of optical communication systems. Short-haul microwave radios may require governmental licensing, whereas optical communications system usually do not. Thus, a need exists for an optical receiving system that can compensate for attenuation from atmospheric conditions.
Free-space optical communications are subject to scintillation. Scintillation refers to an undesired variation in the received intensity of an electromagnetic signal propagating through a path of varying air density. Because air density is not entirely uniform over a cross-sectional area or volume of a propagating beam, the path through which the electromagnetic signal propagates may be modeled as a series of air pockets of varying density. The changes in density at the transitions between the air pockets may refract the electromagnetic signal causing attenuation or redirection of the electromagnetic signal. Scintillation can lead to unwanted fading of the electromagnetic signal and a reduction of the maximum reliable path length for an optical communications system.
Thus, a need exists for a receiving system that reduces or eliminates the deleterious effects of scintillation.
In conventional receiving systems, the cross-sectional area of the electromagnetic signal is on the order of one meter in diameter and the collection lens or mirror is merely five to ten centimeters in diameter because larger high quality lens are prohibitively expensive. While a large Fresnel lens is not prohibitively expensive, Fresnel lenses have not been used in commercially available optical receiving systems because a Fresnel lens alone does not generally provide an adequate focal spot for a detector, which has a response time sufficient for Megabit per second baud communications rates or greater. Thus, a need exists for facilitating the use of a commercially viable, larger lens in optical receiving systems.
In accordance with the invention, an optical receiving system comprises a Fresnel lens optically coupled to a detector via a tapered concentrator. The Fresnel lens is adapted to receive an electromagnetic signal and has a Fresnel focal point. The tapered concentrator has a first end surface area larger than a second end surface area. The detector has a sensing surface area oriented to receive the electromagnetic signal emerging from the tapered concentrator.
The combination of the Fresnel lens and the tapered concentrator cooperates such that. the tapered concentrator accepts the focal spot of the Fresnel lens and appropriately concentrates the focal spot onto the detector. The optical receiving system may reduce the effects of scintillation, attenuation, or both by collecting electromagnetic energy over majority of the cross-sectional area of the electromagnetic signal (e.g., conical beam) at the Fresnel lens.
One aspect of the invention includes a Fresnel lens with an aperture sufficient to reduce or eliminate the effect of scintillation upon the propagation of the electromagnetic signal incident upon the Fresnel lens. Another aspect of the invention includes a tapered concentrator that reduces a first focused radiation pattern produced by the Fresnel lens to a second focused radiation pattern having a localized area of intensity commensurate with a sensing surface area of a detector.